1. Field of the Invention
The present invention relates to a focal-plane shutter device for cameras.
2. Description of Related Art
Heretofore, for many cameras, there has been widely employed a focal-plane shutter device in which a plurality of arms are used to constitute a parallel link mechanism and shutter blades (a leading shutter blade and a trailing shutter blade) for forming a slit are fitted to fore ends of the arms. In such a focal-plane shutter device, the leading shutter blade, which has been covering a shutter opening, travels toward an end of the shutter opening to expose the shutter opening, and, after the lapse of a predetermined period of time according to a shutter speed, the trailing shutter blade, which has been opening the shutter opening, travels toward the end of the shutter opening to cover the shutter opening, so that a slit defined between the leading shutter blade and the trailing shutter blade shifts from one end of the shutter opening to the other end thereof to supply necessary exposure light to a photographing image plane of film.
Such a focal-plane shutter device is carefully manufactured in such a manner as to make uniform the movement characteristics of the leading shutter blade and the trailing shutter blade, particularly the movement characteristics of slit-forming edges thereof for exposure, for the time period from the exposure start to the exposure end, in order to prevent the occurrence of the unevenness of exposure over the whole photographing image plane. In addition, the conventional focal-plane shutter device is manufactured on the assumption that the slit-forming edges move in parallel with each other in accordance with the driving of both the shutter blades by the parallel link mechanism.
However, a play (fitting gap) is formed in a fitting section between the fore end of the arm and the shutter blade or a fitting section between a base end of the arm and a base plate of the shutter, so as to make smooth movement of each shutter member. For this reason, even if the shutter blades are driven by using the parallel link mechanism, practically the slit-forming edges of the shutter blades do not move in parallel with each other, so that the exposure in the longitudinal directions (traverse directions in the case of a vertical-travel-type focal-plane shutter device) of the slit cannot be made uniform, thus leading to the unevenness of exposure.
Here, referring to FIGS. 3A to 3C, FIGS. 4A to 4C and FIGS. 5A to 5C, a description will be made hereinbelow of a mechanism for the occurrence of the unevenness of exposure in transverse directions. FIGS. 3A to 3C are illustrations of an ideal condition of no unevenness of exposure in transverse directions in the case of a vertical-travel-type focal-plane shutter device. Although the leading shutter blade and the trailing shutter blade originally take an accelerated motion, the description will be taken on the assumption that each of the shutter blades takes a uniform-speed motion for simplicity.
In FIGS. 3A, 3B and 3C, reference numeral 101 denotes a shutter opening provided in a shutter base plate (not shown), and reference numerals 102 and 103 denote a main arm and a sub-arm for travel-driving a leading shutter blade 104, respectively. Base end portions of the main arm 102 and the sub-arm 103 are rotatably fitted to the shutter base plate in such a way that shafts 105 and 106 mounted on the shutter base plate are fitted into holes 102a and 103a, respectively. Further, the leading shutter blade 104 is rotatably fitted to the fore end portions of the main arm 102 and the sub-arm 103 in such a way that dowels 107 and 108 mounted on the leading shutter blade 104 are fitted into holes 102b and 103b, respectively.
The distance between the rotating shafts on the side of the base end portions of the main arm 102 and the sub-arm 103 (the distance between the shaft 105 and the shaft 106) is equal to the distance between the fitting shafts of the leading shutter blade 104 on the side of the fore end portions of the main arm 102 and the sub-arm 103 (the distance between the dowels 107 and 108), and a parallel link is constructed by the main arm 102 and the sub-arm 103.
The leading shutter blade 104, which is one of a plurality of light-shielding blades, is a member for forming a slit for exposure.
On the other hand, reference numerals 109 and 110 denote a main arm and a sub-arm for travel-driving a trailing shutter blade 111, respectively. Base end portions of the main arm 109 and the sub-arm 110 are rotatably fitted to the shutter base plate in such a way that shafts 112 and 113 mounted on the shutter base plate are fitted into holes 109a and 110a, respectively. Further, the trailing shutter blade 111 is rotatably fitted to the fore end portions of the main arm 109 and the sub-arm 110 in such a way that dowels 114 and 115 mounted on the trailing shutter blade 111 are fitted into holes 109b and 110b, respectively.
The distance between the rotating shafts on the side of the base end portions of the main arm 109 and the sub-arm 110 (the distance between the shaft 112 and the shaft 113) is equal to the distance between the fitting shafts of the trailing shutter blade 111 on the side of the fore end portions of the main arm 109 and the sub-arm 110 (the distance between the dowels 114 and 115), and, thus, a parallel link is constructed by the main arm 109 and the sub-arm 110.
The trailing shutter blade 111, which is one of the plurality of light-shielding blades, is a member for forming a slit for exposure.
In the ideal type shown in FIGS. 3A, 3B and 3C, no fitting play (fitting gap) exists in each of the fitting sections, that is, between each of the holes 102a, 103a, 102b, 103b and the corresponding one of the shafts 105, 106 and dowels 107, 108 fitted therein or between each of the holes 109a, 110a, 109b, 110b and the corresponding one of the shafts 112, 113 and dowels 114, 115 fitted therein. Accordingly, from a state near the exposure start shown in FIG. 3A through a state in process of exposure shown in FIG. 3B up to a state near the exposure end shown in FIG. 3C, slit-forming edges 104a and 111a of the leading shutter blade 104 and the trailing shutter blade 111 are kept to be in parallel with each other, thus producing no unevenness of exposure in the transverse directions.
However, in the case of the actual products of focal-plane shutter devices, since the fitting play is made in the fitting sections as mentioned before, as shown in FIGS. 4A to 4C and in FIGS. 5A to 5C, difficulty is encountered to maintain the slit-forming edges 104a and 111a of the leading shutter blade 104 and the trailing shutter blade 111 to be in parallel with each other, which causes the unevenness of exposure in the transverse directions.
FIGS. 4A, 4B and 4C show the variation in the inclination of the slit-forming edge 104a of the leading shutter blade 104. In a state near the exposure start shown in FIG. 4A, when a driving force is transferred from a drive lever (not shown) engaging with a driving force transmission hole 102c of the main arm 102 to the main arm 102, the main arm 102 receives a force to rotate clockwise around the shaft 105, with the result that a force indicated by an arrow F1 works on the dowel 107 of the leading shutter blade 104, which is fitted in the hole 102b of the main arm 102 with a fitting play existing therebetween.
When the force indicated by the arrow F1 works, the leading shutter blade 104 tends to stay at that position due to the large-load static frictional force and the inertia force thereof about the center of gravity G1. In addition, due to the load (inertial force) of the sub-arm 103, a force for causing the leading shutter blade 104 to rotate counterclockwise works on the dowels 107 and 108. For this reason, owing to the play within the link system, the leading shutter blade 104 (slit-forming edge 104a) starts to travel, in an inclined state with its right-hand section raised with respect to the end portion of the shutter opening 101, as shown in FIG. 4A.
Then, in a state in process of exposure shown in FIG. 4B, the force for causing the shutter leading blade 104 to rotate generated by the load of the sub-arm 103 due to the force working from the main arm 102 onto the dowel 107 balances with the centrifugal force working on the leading shutter blade 104, so that the leading shutter blade 104 (slit-forming edge 104a) travels in a state of being in parallel with the end portion of the shutter opening 101. Incidentally, with this state as the boundary, the force working on the leading shutter blade 104 changes from the counterclockwise force to the clockwise force.
Subsequently, in a state near the exposure end shown in FIG. 4C, the force for causing the leading shutter blade 104 to rotate clockwise working from the sub-arm 103 onto the dowel 108 due to the force of the arrow F1 working from the main arm 102 onto the dowel 107 (i.e., the force occurring by the load of the sub-arm 103), and the centrifugal force working clockwise on the leading shutter blade 104 are given to the leading shutter blade 104. Therefore, owing to the play within the link system, the leading shutter blade 104 (slit-forming edge 104a) terminates the travelling, in an inclined state with its right-hand section lowered.
FIGS. 5A, 5B and 5C show the variation in the inclination of the slit-forming edge 111a of the trailing shutter blade 111. In a state near the exposure start shown in FIG. 5A, when a driving force is transferred from a drive lever (not shown) engaging with a driving force transmission hole 109c of the main arm 109 to the main arm 109, the main arm 109 receives a force to rotate clockwise around the shaft 112, with the result that a force indicated by an arrow F2 works on the dowel 114 of the trailing shutter blade 111, which is fitted in the hole 109b of the main arm 109 with a fitting play existing therebetween.
When the force indicated by the arrow F2 works, the trailing shutter blade 111 tends to stay at that position by the static frictional force and the inertial force of the trailing shutter blade 111 about the center of gravity G2, and hence, a force to rotate the trailing shutter blade 111 counterclockwise works on the dowel 114 of the trailing shutter blade 111. However, when the force indicated by the arrow F2 works, due to the load (inertia force) of the sub-arm 110, a force occurs to rotate the trailing shutter blade 111 clockwise about the dowel 115 of the trailing shutter blade 111, thereby offsetting the above counterclockwise rotating force. Therefore, even if the fitting play exists within the link system, the trailing shutter blade 111 (slit-forming edge 111a) starts to travel, in a state of being in parallel with the end portion of the shutter opening 101.
Then, in a state in process of exposure shown in FIG. 5B, since a balance is kept among the force working from the main arm 109 onto the dowel 114, the force generated by the load of the sub-arm 110 to rotate the trailing shutter blade 111 and the centrifugal force working on the trailing shutter blade 111, the trailing shutter blade 111 (slit-forming edge 111a) travels in a state of being in parallel with the end portion of the shutter opening 101.
Subsequently, in a state near the exposure end shown in FIG. 5C, although a force to rotate the trailing shutter blade 111 counterclockwise working on the dowel 115 from the sub-arm 110 due to the force of the arrow F2 working from the main arm 109 onto the dowel 114 (i.e., the force generated by the load of the sub-arm 110) is given to the trailing shutter blade 111, it is offset by the centrifugal force for causing the trailing shutter blade 111 to rotate clockwise. For this reason, even if a fitting play exists within the link system, the trailing shutter blade 111 (slit-forming edge 111a) terminates the travelling, in a state of being in parallel with the end portion of the shutter opening 101.
As described above, the slit-forming edge 104a of the leading shutter blade 104 takes an inclined state with its right-hand section raised with respect to the end portion of the shutter opening 101 in the vicinity of the travel start, becomes parallel with the end portion of the shutter opening 101 in the middle of the traveling, and assumes an inclined state with its right-hand section lowered in the vicinity of the travel end. On the other hand, the slit-forming edge 111a of the trailing shutter blade 111 is substantially in parallel with the end portion of the shutter opening 101 from the travel start up to the end. Therefore, a left and upper portion and a right and lower portion of the photographing image plane result in the overexposure condition with respect to a central portion thereof.
Further, although the unevenness of exposure thus occurring does not create a big problem in the case of a conventional low-speed shutter in which a slit width is made relatively wide, it causes a serious problem in the case of a high-speed shutter in which a slit width is made considerably narrow (whose maximum speed is 1/8000 seconds or higher).